Machine Design

The basics of pneumatic control valves

Directional-control valves are vital in any pneumatic circuit, directing or blocking airflow to control the speed or sequence of operations. One method of classifying directional-control valves is by the flow paths under various operating conditions. Important factors are the number of possible valve positions and the number of ports and flow paths. Here are some basic configurations.

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Two-way, two-position valves consist of two ports connected with a passage that can be opened or blocked to control flow through the valve. Usually, an electrically activated solenoid shifts the valve spool or poppet to direct flow. The valve provides an easy on-off function, which many systems use to interlock, isolate, and connect various system parts.
Three-way, two-position valves consist of three ports connected through passages within a valve body. The valves pressurize and exhaust one outlet port to control a single-acting cylinder, or pilot another valve.

Three-way valves direct pressurized air to the cap-end side of the cylinder. Shifting the spool to the other extreme position blocks flow and pressure to the actuator. The actuator is connected to the exhaust passage, therefore spring force or gravity must return the rod to its original position.

Three-way valves may also be used in pairs to operate a doubleacting cylinder, thus replacing a four-way valve. Consider using paired three-way valves instead of a four-way when high cylinder speeds are necessary. Close coupling of three-way valves to the cylinder ports reduces cylinder back pressure and pressure drop in the lines, permitting higher cylinder velocities. The valves are also used to save compressed air in highly cyclic applications or when intermediate positions are required.

Four-way, two-position valves have four or five ports with two or three positions. A two-position valve has two distinct flow paths in each position to actuate and reverse cylinders, rotary actuators, or bidirectional motors. The spool directs flow from the pressure port while the other actuator port exhausts to atmosphere at the same time.

The five-port version of the two-position, four-way valve has five individual ports which provide various valve configurations such as two different exhaust or pressure ports. In the latter case, for example, the valve might supply high pressure to actuate a cylinder and provide high clamping force, but deliver lower pressure (from a regulator) to release the clamp. Using lower pressure conserves energy.

Four-way, three-position valves are also available in five-port versions. The most common center conditions for these valves are "all ports blocked" and "exhaust center." The two extreme positions of the four-port, four-way valve are the power positions of the valve which control actuator movement. The center position is designed to satisfy some system requirement such as locking an actuator or letting it float.

This is just a brief look at directional valves and other factors, such as the basic valve design, should be considered when constructing pneumatic systems.

Information for this article provided by the Pneumatics Div. of Parker Hannifin, Richland, Mich. For more details, visit

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